Around 1970 Luxman company came with
its first "high quality" integrated amplifier on the European market. This
amplifier was a modern design with excelent characteristics. It had a full
complementary output stage and a differential input stage of the power
amplifier. It was fully symmetric supplied and the power supply stage was
generously built.
The amplifier was very well built and specially the power amplifier stages were
an example of compact and slick electronic design and good amplifier
building practices.
The 505X is a lower powered version
of the 507X.

Special
futures:

The most remarkable thing
about this amplifier is that the driver- and power- transistors were also
directly soldered to the poweramp-pcb: so no extra wiring was needed. That was
quite remarkable in these days. The specifications were very good as you can see
at the picture below. It was one of the first Japanese transistor amplifiers
starting a new audio era. It incorporates the first generation transistors
designed in Japan specially for audio purposes. Later Luxmans were built with
transistors specially designed for audio. This Luxman SQ 507 amp was built to be a fast and powerful
audio machine with good speaker control. It
had good tone-control possibilities on front.

The schematic diagram shows
a 3 stage amplifier: RIAA amp, control amp, output power amp.
The RIAA amp is a
DC-coupled 3 transistor amp with low noise PNP silicon transistors, with RIAA correction network in the
NFB, it is of quite conventional design. It is capable of
taking input signals of up to x100 above the average phono-cartridge levels
without causing overload problems.
The
tone control unit consists of 2 stages, plus buffer stage and allows extensive tone control, with
the control taking action on selectable frequencies. The tone control can be
switched off by selecting the "defeat" position.
The Power amp is a DC
coupled class AB amp with differential input stage and a full complementary OTL
/ OCL output stage, a compact and modern design. NFB is moderate. It is fully symmetric supplied.
The power
supply stage is generously built, with a stabilized section for the preamps.
Distortion and noise levels of this amp are low.
The transistors used were modern high
speed transistors specially made by NEC and Toshiba in Japan for audio
applications.
Reliability of this design appeared to be very high. There is no
serious speaker protection employed in this design.

The
power amplifier module: copper side of the pcb, with the heatsink directly
mounted to the pcb.

The
power amplifier module turned around to the component side.

A not
very successfull repair of such a module.

another version of drivers.

The legendary 2SC1161 of NEC, several different types were used by
Lux.

Schematic diagram of this power module. ^

Sound:

The sound of this amp is
good and stable. A pleasure to listen to.

Modifications:

This amp is greatly DC-coupled, unfortunately the coupling capacitors are on
some places not of high quality (tantallum) and started to give problems in the
control amp section after several years, due to DC-leakage through the variable
resistors and switches. This problem could easely be solved by replacing the
capacitors by polypropylene's of the same value and voltage.
Also the low cut switch can easely be modified into a subsonic filter switch, by
modifying the capacitors of the low cut section. A subsonic filter is
indispensable when playing vinyl records.
The power supply main capacitors value can extended together with the rectifier
diodes to make the supply even more rugged that it already is, the low
frequencies benefit from this, 10.000 uF same voltage is a good value, splitting
up the main psu is also a good idea: this is using 2 diode sections and 2
capacitor sections, for each power amp one.
For better speaker control the speaker switch can be omitted and hardwired with
good quality stranded copper wire, the low frequencies and the sound image
benefit from this.

Photo: Arthur de la Court

This is a picture of a faulty
electrolytic capacitor that was found in the power suppy unit (psu) of a Luxman
SQ507X. As you can see it is heavely corroded and got very hot, one connection
lead is completely missing, it was corroded away by the acids coming out of the
cap as it started leaking. The cap did not function anymore and consequently the
psu started causing hum in the sound of the amp.